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United States Patent |
6,039,789
|
McMullen
,   et al.
|
March 21, 2000
|
Removal of boron and fluoride from water
Abstract
A process for reducing boron and/or fluoride ion content of water. Feed
water is contacted, in the presence of magnesium, with an alkaline
hydroxide to produce treated water and a magnesium precipitate containing
boron and fluorine. The precipitate is separated from the treated water.
The boron content of water is reducible from above about 0.8 mg/L to below
about 0.7 mg/L, and the fluoride ion content is reducible from above about
1 mg/L to below about 0.9 mg/L. The magnesium precipitate is optionally
used to neutralize pressure oxidized ore slurry or roaster calcine in the
context of gold recovery operations.
Inventors:
|
McMullen; Jacques (Toronto, CA);
Tsu; Wilson (Elko, NV);
Kargel; Reinhard (Toronto, CA)
|
Assignee:
|
Barrick Gold Corporation (Ontario, CA)
|
Appl. No.:
|
049331 |
Filed:
|
March 27, 1998 |
Current U.S. Class: |
75/711; 75/744 |
Intern'l Class: |
C22B 003/12 |
Field of Search: |
75/744,711
|
References Cited
U.S. Patent Documents
4035469 | Jul., 1977 | Richmond et al. | 423/164.
|
4201758 | May., 1980 | Allain et al. | 423/497.
|
4311679 | Jan., 1982 | Queneau et al. | 423/55.
|
4384889 | May., 1983 | Wiewiorowski et al.
| |
4402835 | Sep., 1983 | Mattera et al. | 210/724.
|
4497781 | Feb., 1985 | Spoors et al. | 423/164.
|
4571263 | Feb., 1986 | Weir et al. | 75/744.
|
4619814 | Oct., 1986 | Salter et al. | 423/27.
|
5071477 | Dec., 1991 | Thomas et al. | 75/744.
|
5401296 | Mar., 1995 | Martenson et al. | 75/741.
|
5489326 | Feb., 1996 | Thomas et al. | 75/744.
|
5536297 | Jul., 1996 | Marchbank et al. | 75/744.
|
Foreign Patent Documents |
53-46163 | Apr., 1978 | JP.
| |
57-071692 | May., 1982 | JP.
| |
59-166290 | Sep., 1984 | JP.
| |
1560484 | Apr., 1990 | UA.
| |
Other References
Okay, et al., Boron Pollution in the Simav River, Turkey and Various
Methods of Boron Removal; Water Res. vol. 19, No. 7, pp. 857-862, 1985.
Wong, Boron Control in Power Plant Reclaimed Water for Potable Reuse;
Environmental Progress, vol. 3, No. 1, pp. 5-11, Feb. 1984.
Kawamura, Integrated Design of Water Treatment Facilities, Wiley & Sons
(1991).
Idelovitch et al, Magnesium Recycling by Carbonation and Centrifugation of
High-lime Wastewater Sludge, Journal WPCF, vol. 55, No. 2, pp. 136-144
(XP-002092114) Feb. 1983.
Turek, The Influence of Magnesium Hydroxide Precipitation Conditions on the
Boron Content, Polish Journal of Applied Chemistry, 1995, pp. 211-213,
(XP-002092123) No Month.
|
Primary Examiner: Andrews; Melvyn
Assistant Examiner: McGuthry-Banks; Tima
Attorney, Agent or Firm: Senniger, Powers, Leavitt & Roedel
Claims
What is claimed is:
1. An integrated process for reducing boron content of feed water, and
recovering gold from a refractory auriferous ore containing sulfide
sulfur, the process comprising:
contacting said feed water containing boron in the presence of a source of
magnesium with an alkaline hydroxide to produce treated water and a
magnesium precipitate containing boron;
separating the treated water and precipitate into a substantially liquid
fraction comprising water having a reduced boron content and a
substantially solid fraction comprising said precipitate;
forming an aqueous ore slurry comprising said refractory auriferous ore;
subjecting the aqueous ore slurry to pressure oxidation in an autoclave to
produce an oxidized ore slurry;
raising the pH of the oxidized ore slurry by contacting said oxidized ore
slurry with a quantity of said substantially solid fraction comprising
said magnesium precipitate; and
recovering gold from said oxidized ore slurry.
2. The process of claim 1 wherein said feed water contains fluoride ions,
wherein said magnesium precipitate contains fluorine, and wherein said
treated water has a reduced fluoride ion content.
3. An integrated process for reducing fluoride ion content of feed water,
and recovering gold from a refractory auriferous ore containing sulfide
sulfur, the process comprising:
contacting said feed water containing fluoride ion in the presence of a
source of magnesium with an alkaline hydroxide to produce treated water
and a magnesium precipitate containing fluorine;
separating the treated water and precipitate into a substantially liquid
fraction comprising water having a reduced fluoride ion content and a
substantially solid fraction comprising said precipitate;
forming an aqueous ore slurry comprising said refractory auriferous ore;
subjecting the aqueous ore slurry to pressure oxidation in an autoclave to
produce an oxidized ore slurry;
raising the pH of the oxidized ore slurry by contacting said oxidized ore
slurry with a quantity of said substantially solid fraction comprising
said magnesium precipitate; and
recovering gold from said oxidized ore slurry.
4. An integrated process for reducing boron content of feed water, and
recovering gold from a refractory auriferous ore containing sulfide
sulfur, the process comprising:
contacting said feed water containing boron in the presence of a source of
magnesium with an alkaline hydroxide to produce treated water and a
magnesium precipitate containing boron;
separating the treated water and precipitate into a substantially liquid
fraction comprising water having a reduced boron content and a
substantially solid fraction comprising said precipitate;
subjecting said auriferous ore to roasting in a roaster to produce a
roaster calcine;
forming an aqueous slurry comprising said roaster calcine;
raising the pH of the calcine by contacting said calcine with a quantity of
said substantially solid fraction comprising said magnesium precipitate;
and
recovering gold from said calcine.
5. The process of claim 4 wherein said feed water contains fluoride ions,
wherein said magnesium precipitate contains fluorine, and wherein said
treated water has a reduced fluoride ion content.
6. An integrated process for reducing fluoride ion content of feed water,
and recovering gold from a refractory auriferous ore containing sulfide
sulfur, the process comprising:
contacting said feed water containing fluoride ion in the presence of a
source of magnesium with an alkaline hydroxide to produce treated water
and a magnesium precipitate containing fluorine;
separating the treated water and precipitate into a substantially liquid
fraction comprising water having a reduced fluoride ion content and a
substantially solid fraction comprising said precipitate;
subjecting said auriferous ore to roasting in a roaster to produce a
roaster calcine;
forming an aqueous slurry comprising said roaster calcine;
raising the pH of the calcine by contacting said calcine with a quantity of
said substantially solid fraction comprising said magnesium precipitate;
and
recovering gold from said calcine.
7. An integrated process for reducing boron and fluoride ion content of
feed water, and recovering gold from a refractory auriferous ore
containing sulfide sulfur, the process comprising:
contacting said feed water containing greater than 0.8 mg/L boron and
greater than 1 mg/L fluoride ion at a temperature in the range of about
85.degree. F. to about 130.degree. F. with a source of magnesium in a
dosage of between about 10 and about 80 mg magnesium per liter of feed
water in the form of a magnesium sulfate in solution in a contacting zone;
contacting said feed water with a first lime slurry to produce treated
water and a magnesium precipitate containing
3MgO.Mg(OH).sub.2.6SiO.sub.2.6H.sub.2 O, Mg(OH).sub.2, B, and F;
separating the treated water and precipitate into a substantially liquid
fraction comprising water having a boron content below 0.7 mg/L and
fluoride ion content below 0.9 mg/L and a substantially solid fraction
comprising said magnesium precipitate;
transferring a first quantity of said solid fraction to the contacting zone
to facilitate nucleation of said magnesium precipitate;
forming an aqueous ore slurry comprising said refractory auriferous ore;
subjecting the aqueous ore slurry to pressure oxidation in an autoclave to
produce an oxidized ore slurry;
raising the pH of the oxidized ore slurry to between about 3 and about 4 by
contacting said oxidized ore slurry with a second quantity of said
substantially solid fraction comprising said magnesium precipitate to
produce an intermediate oxidized slurry;
raising the pH of the intermediate oxidized slurry further to between about
10 and about 10.5 by contacting said intermediate oxidized slurry with a
second lime slurry to produce a neutralized oxidized slurry; and
recovering gold from said neutralized oxidized slurry.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for removing boron and fluoride ions
from water generated in mine dewatering operations so the ultimate boron
and fluoride levels meet stringent governmental environmental requirements
for discharge of such water into surface streams.
As a particular example, water resulting from dewatering operations at gold
mines in the Carlin trend area of Nevada, U.S.A. can contain dissolved
boron concentrations on the order of about 1 mg/L, which due to
environmental regulations must be reduced to about 0.6 mg/L or less before
such water can be dispensed into the Humboldt River. Such water may also
contain dissolved fluoride ion concentrations on the order of about 1.5
mg/L, which must be reduced to about 0.8 mg/L or less.
Prior attempts to remove boron from water by conventional water treatment
methods such as treatment with aluminum sulfate, ferric salts, and lime
have proved to be ineffective. Evaporation-crystallization processes and
solvent extraction processes have been investigated. Ion exchange
processes employing strong base ion exchange resins have been demonstrated
to be effective, but remove other ionic species as well. As such, they are
inefficient if boron is the only element to be removed. One resin, the ion
specific resin for boron developed by Rohm & Haas Co., is also uneconomic
for treatment of large volumes of water containing a low B concentration.
Mine dewatering operations are found in conjunction with, for example, gold
recovery operations in the Carlin trend area of Nevada, U.S.A. Processes
for the recovery of gold from refractory sulfidic ores, "double
refractory" ores containing sulfidic and carbonaceous material, and other
difficult ores such as those ores located in the Carlin trend employ
pressure oxidation under acidic conditions as disclosed, for example, in
Thomas et al. U.S. Pat. No. 5,071,477 and Thomas et al. U.S. Pat. No.
5,489,326 and/or, alternatively, a roasting operation. The products of
such oxidation processes typically contain high acidic contents which must
be neutralized before further processing by cyanidation as disclosed in
the Thomas et al. patents, or by thiosulfate leaching in the manner
disclosed in Marchbank et al. U.S. Pat. No. 5,536,297. There is a need,
therefore, for an abundant source of neutralizing agent which is
compatible with the gold recovery process in that it does not contain
contaminants or other agents which substantially interfere with gold
recovery, and which does not add substantially to the raw material
requirements and disposal requirements for the overall operation.
SUMMARY OF THE INVENTION
Among the several objects of the invention, therefore, is to provide a
water treatment process for reducing the boron content of water having a
relatively low boron content down to a level of about 0.6 mg/L or less.
Another object is to provide a process for reducing the fluoride ion
content of water having a relatively low fluoride ion content down to a
level of about 0.8 mg/L or less. Depending on site-specific requirements,
another object is to utilize an alkaline sludge produced in the water
treatment process for partially neutralizing acidic gold ore slurry from
autoclaves or roasters in the context of gold recovery operations,
otherwise the sludge would have to be disposed per applicable regulations.
Briefly, therefore, the invention is directed to a process for reducing
boron and/or fluoride ion content of feed water containing boron. The pH
of the feed water is adjusted in the presence of a source of magnesium in
a concentration of between about 10 and about 80 mg magnesium per liter of
feed water, to produce treated water and a magnesium precipitate
containing boron and/or fluorine. The precipitate is separated from the
treated water such that the treated water contains less than about 2 mg/L
boron and/or less than about 0.9 mg/L fluoride ions. The magnesium is from
a source selected from magnesium present in the feed water, magnesium
added to the feed water in the form of a magnesium salt, and combinations
thereof.
The invention is also directed to a process for reducing boron and/or
fluoride ion content of water by contacting a quantity of feed water
containing boron in the presence of magnesium and silicon with an alkaline
hydroxide to produce treated water and a magnesium precipitate comprising
an alkaline magnesium silicate containing boron and/or fluorine and
separating the precipitate from the treated water such that the treated
water has a reduced boron and/or fluoride ion content.
In another aspect, the invention is directed to a process for reducing
boron and/or fluoride ion content of water, the process comprising
adjusting, in the presence of magnesium, the pH of a quantity of feed
water containing boron and/or fluoride ion at a temperature in the range
of about 85.degree. F. to about 130.degree. F., to between about 10.2 and
about 10.6 to produce treated water and a magnesium precipitate containing
boron and/or fluorine, and separating the precipitate from the treated
water to produce water having a reduced boron and/or fluoride ion content.
The invention is further directed to a process for reducing boron and/or
fluoride ion content of water by adjusting the pH of a quantity of feed
water containing boron in the presence of magnesium by contacting the
water with an alkaline hydroxide in a contacting zone to produce treated
water and a magnesium precipitate containing boron and/or fluorine,
separating the treated water and precipitate into a substantially liquid
fraction comprising treated water having a reduced boron and/or fluoride
ion content and a substantially solid fraction comprising the precipitate,
and transferring a portion of the solid fraction to the contacting zone to
facilitate nucleation of said magnesium precipitate.
In another aspect, the invention is directed to an integrated process for
reducing boron and/or fluoride ion content of water, and recovering gold
from a refractory auriferous ore containing sulfide sulfur. The process
includes contacting a quantity of feed water containing boron and/or
fluoride ion in the presence of magnesium with an alkaline hydroxide to
produce treated water and a magnesium precipitate containing boron and/or
fluorine, and separating the treated water and precipitate into a
substantially liquid fraction comprising water having a reduced boron
and/or fluoride ion content and a substantially solid fraction comprising
the precipitate. An aqueous slurry is formed of the refractory auriferous
ore and subjected to pressure oxidation in an autoclave to produce an
oxidized ore slurry, the pH of the oxidized slurry is raised by contacting
the slurry with the substantially solid fraction containing the magnesium
precipitate, and gold is recovered from the slurry.
The invention is further directed to an integrated process for reducing
boron and/or fluoride ion content of water, and recovering gold from a
refractory auriferous ore containing sulfide sulfur. A quantity of feed
water containing boron and/or fluoride ion is contacted with an alkaline
hydroxide in the presence of magnesium to produce treated water and a
magnesium precipitate containing boron and/or fluorine. The treated water
and precipitate are separated into a substantially liquid fraction
comprising water having a reduced boron and/or fluoride ion content and a
substantially solid fraction comprising the precipitate. A refractory ore
is subjected to roasting in a roaster to produce a roaster calcine. An
aqueous slurry is formed from the roaster calcine, and the pH of the
calcine is raised by contacting the calcine with the substantially solid
fraction containing the magnesium precipitate, and gold is recovered from
the calcine.
The invention is also directed to an integrated process for reducing boron
and fluoride ion content of water, and recovering gold from a refractory
auriferous ore containing sulfide sulfur. In the process a quantity of
feed water containing greater than 0.8 mg/L boron and greater than 1 mg/L
fluoride ion at a temperature in the range of about 85.degree. F. to about
130.degree. F. is contacted in a contacting zone with magnesium in a
dosage of between about 10 and about 80 mg magnesium per liter of feed
water in the form of a magnesium sulfate in solution. The feed water is
contacted with a first lime slurry to produce treated water and a
magnesium precipitate containing 3MgO.6SiO.sub.2.Mg(OH).sub.2.6H.sub.2 O,
Mg(OH).sub.2, B, and F. The treated water and precipitate are separated
into a substantially liquid fraction comprising water having a boron
content below 0.7 mg/L and fluoride ion content below 0.9 mg/L and a
substantially solid fraction the precipitate. A portion of the solid
fraction is transferred to the contacting zone to facilitate nucleation of
the magnesium precipitate. An aqueous ore slurry is formed from the
refractory auriferous ore, and subjected to pressure oxidation in an
autoclave to produce an oxidized ore slurry. The pH of the oxidized slurry
is raised to between about 3 and about 4 by contacting the slurry with the
substantially solid fraction containing the magnesium precipitate. The pH
of the oxidized slurry is raised further to between about 10 and about
10.5 by contacting the slurry with a second lime slurry, and gold is
recovered from the slurry.
Other objects and features of the invention will be in part apparent and in
part pointed out hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a flowsheet describing the principal operations in accordance
with the process of the invention.
FIG. 1A is a flowsheet describing the principal full scale operations in
accordance with the water treatment circuit of the invention.
FIGS. 2 to 12 present data generated by the working examples.
FIGS. 13 to 16 are photomicrographs of sludge particles produced by the
water treatment process of the invention.
FIG. 17 is a schematic representation of the sludge particles produced by
the water treatment process of the invention.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the process of the invention, magnesium salt (e.g.,
MgSO.sub.4) in solution and calcium hydroxide Ca(OH).sub.2 slurry are
successively added to water to be treated under conditions of pH and
temperature effective to precipitate magnesium compounds. Boron and
fluoride ions are removed by co-precipitation, adsorption and/or
enmeshment with what appears to be an alkaline magnesium silicate with a
quantity of Mg(OH).sub.2. In one preferred embodiment, the boron content
is reduced from above about 0.8 mg/L to below about 0.7 mg/L, and the
fluoride ion content is reduced from above about 1 mg/L to below about 0.9
mg/L. In a still further preferred embodiment, the boron and fluoride ion
contents are reduced to less than 75% of their original level. To dispose
of the precipitate, precipitated solids are separated from the water by
gravity settling in a clarifier. A certain amount of solids is recycled to
the reaction tank to initiate and accelerate the precipitation reactions,
and to improve settling characteristics in the clarifier. The amount of
solids to be discarded is further processed for disposal or use in other
unit operations. The liquid overflow from the clarifier is optionally
filtered to further reduce its solids content, and neutralized with acid
to meet environmental effluent pH guidelines.
In one preferred embodiment of the invention, water having the
characteristics stated in Table 1 is treated to provide water having the
characteristics stated in Table 2.
TABLE 1
______________________________________
Range Average
______________________________________
Boron mg/L 0.73-1.00
0.84
Fluoride mg/L 1.18-1.78
1.35
Magnesium mg/L 20.2-23.8
22.3
Silica mg/L 19.6-21.5
20.9
TDS mg/L 520
pH 8.3507-8.52
Temperature, .degree. F.
86-132 100
Alkalinity
Total as 371
CaCO.sub.3 mg/L
Hardness Total 272
as CaCO.sub.3 mg/L
______________________________________
TABLE 2
______________________________________
Boron mg/L 0.61 max.
Fluoride mg/L 0.87 max.
TDS mg/L 358 max
pH 7.5-8.3
Temperature within 2 .degree.C. ambient river
temp.
______________________________________
Magnesium salt in solution is added to the feed water before entering the
reactor. The preferred salt is the sulfate salt in anhydrous form
(MgSO.sub.4) or hydrated form (MgSO.sub.4.7H.sub.2 O). Other suitable
sources of Mg include, but are not limited to, MgCl.sub.2,
MgCl.sub.2.6H.sub.2 O, and Mg(HCO.sub.3).sub.2. It is preferred that the
Mg salt be added in its dissolved state, to facilitate rapid and uniform
distribution of the salt in the water. The amount of magnesium (Mg) salt
added depends on the concentration of Mg present in the feed water, the
concentration of boron and fluoride in the feed water, the amount of boron
and fluoride to be removed, the treatment pH, and the amount of silica in
the feed water. For a typical feed water having the analysis shown in
Table 3, between about 10 and about 80 mg/L Mg, preferably about 20 mg/L
Mg, is added in accordance with this invention.
TABLE 3
______________________________________
Boron 0.85 mg/L
Fluoride 1.4
mg/L
Magnesium 20 mg/L
Silica 39
mg/L
TDS mg/L25-600
pH 6.7-8.2
Temperature 38-55.degree.
C.
Target boron 0.6 mg/L
Target fluoride
0.8 mg/L
Target TDS 358 mg/L
______________________________________
The concentration of Mg to be added is determined by jar tests. The amount
of MgSO.sub.4 added is sufficiently low that the SO.sub.4 ions remain in
solution and no CaSO.sub.4 (Ca being added as a source of hydroxide, as
described below) precipitates. Further, the amount of SO.sub.4 added is
selected so as not to adversely affect the total dissolved solids content
of the water, a significant consideration in view of environmental
requirements for discharge into surface streams.
A source of hydroxide to raise the pH and facilitate the precipitation of
magnesium compounds is added to the stirred tank reactor. The preferred
source is Ca(OH).sub.2 produced by hydrating lime according to the
following reaction:
CaO+H.sub.2 O.fwdarw.Ca(OH).sub.2
Other sources include commercially available hydrated lime [Ca(OH).sub.2 ],
alkaline earth metal hydroxides, as well as dolomitic lime. The amount of
Ca(OH).sub.2 added is determined by titration to achieve the desired
treatment pH. The amount of Ca(OH).sub.2 added is not determined by a
stoichiometric relationship with MgSO.sub.4, as simple stoichiometric
addition of Ca(OH).sub.2 does not guarantee enough (OH).sup.- ion
concentration to cause Mg(OH).sub.2 to precipitate. In one preferred
embodiment where the total dissolved solids of the feed water is 520 mg/L,
420 mg/L Ca(OH).sub.2 is preferred in order to achieve the desired
treatment pH of 10.4 to 10.6. The hydrated lime is added in slurry form to
precipitate calcium carbonate and magnesium hydroxide according to the
following reactions:
CO.sub.2 +Ca(OH).sub.2 .fwdarw.CaCO.sub.3 .dwnarw.+H.sub.2 O
Ca(HCO.sub.3).sub.2 +Ca(OH).sub.2 .fwdarw.2CaCO.sub.3 .dwnarw.+2H.sub.2 O
Mg(HCO.sub.3).sub.2 +Ca(OH).sub.2 .fwdarw.CaCO.sub.3 .dwnarw.+MgCO.sub.3
+2H.sub.2 O
MgCO.sub.3 +Ca(OH).sub.2 .fwdarw.CaCO.sub.3 .dwnarw.+Mg(OH).sub.2 .dwnarw.
Calcium sulfate (CaSO.sub.4) will not precipitate since prevailing
concentrations will normally be well below its solubility limit (2980 mg/L
at 20.degree. C. in pure water). This is acceptable since precipitation of
CaSO.sub.4 is not a requirement for the reduction of the levels of boron
and fluoride.
It is believed that the order of addition, namely, MgSO.sub.4 before
Ca(OH).sub.2 is important, and that simply adding Mg(OH).sub.2 to the feed
water without precipitation reaction thereof in the reactor will not
achieve the desired results.
The pH selected is not, in itself, of narrowly critical. The objective is
to operate at a pH level that will result in the maximum amount of
Mg(OH).sub.2 precipitation, consistent with the constraints of other
parameters that may be imposed upon the system, since the co-precipitation
of Mg(OH).sub.2 along with silicon is the driving force for the reduction
in the levels of boron and fluoride. The treatment pH is selected in order
to remove maximum amounts of magnesium and silica, which results in the
removal of maximum amounts of boron and fluoride, while remaining at level
to ensure the TDS (total dissolved solids) in the final effluent are
within regulatory guidelines. The preferred pH for treating waters
generated by dewatering operations in the Carlin trend is in the range of
about 10.4 to about 10.6. A pH significantly below about 10.2 is avoided
because insufficient Mg(OH).sub.2 may precipitate which would affect the
efficiency of removal of boron and fluoride. A pH significantly above
about 10.8 is avoided because at high pH there is a tendency for increase
in level of TDS due to excess alkalinity [Ca(OH).sub.2 ]. Furthermore, at
too high a pH there is a tendency for boron and fluoride to re-dissolve in
the sludge phase.
The temperature of the feedwater can affect the process parameters, because
at a higher temperature Mg(OH).sub.2 tends to precipitate at a lower pH.
For the typical feed water of the invention, the water temperature is in
the range of about 38.degree. C. to 55.degree. C. At lower temperatures,
for example, 10.degree. C. to 20.degree. C., a pH of about 11 or greater
is required to precipitate magnesium compounds. As such, it is important
to take temperature into account when selecting specific parameters.
In selecting specific parameters a balance is struck among pH, temperature
and magnesium dosage. In particular, the parameters selected take into
account that a higher pH facilitates complete magnesium compound
precipitation and therefore reduces magnesium requirements, while also
producing, disadvantageously, a higher TDS potentially above environmental
guidelines. At a lower pH higher magnesium concentration is required to
achieve equivalent B and F removal. Furthermore, generally, higher
temperatures require a lower pH than lower temperatures.
It has been discovered in connection with this invention that it is
critical to substantially reduce, via a co-precipitation mechanism, the
silicon concentration of water treated by this method in order to achieve
boron and fluoride removal to the levels desirable. (Although silicon and
other elements such as magnesium and boron are referred to herein in their
elemental form, it should be understood such elements may be in a combined
form, e.g., silicates, in water treated by this invention.) In particular,
while a typical water treated by this method has, for example, about 19-22
mg/L Si, it is noted that adequate reductions in B level are achieved only
if silicon levels are also reduced. Without being bound to a particular
theory, it is believed that a relatively stable spherulitic crystalline
matter, in particular an alkaline magnesium silicate, more particularly
3MgO.6SiO.sub.2.Mg(OH).sub.2.6H.sub.2 O (sepiolite), is precipitated, with
co-precipitation of Mg(OH).sub.2. Boron and fluoride are removed by the
precipitation products, although it has not been conclusively determined
whether the mechanism is co-precipitation, adsorption, or enmeshment.
Turning to FIG. 1, in the preferred operation of the process of the
invention, MgSO.sub.4 at a dose of about 20 mg magnesium per liter of
water to be treated is injected into water to be treated before the water
enters a stirred tank reactor. The water is then fed into a stirred tank
reactor for contacting with lime addition by mechanical agitation. The
flow through the tank is controlled to result in a retention time of about
8 minutes in the reactor. Clarifier underflow solids are recycled and fed
into the tank reactor. This recycle has been discovered to advantageously
serve as seed material for initiation of the precipitation reactions. This
recycle also results in precipitates with larger and denser particles
which have faster settling rates and produce a denser sludge. It is
preferred that the recycle be controlled to maintain about 2% to about 8%
solids by volume. In one particularly preferred embodiment, solids in the
first tank are controlled in this manner to be about 2.2% by weight. At
laboratory scale, it is possible to operate the process without this
recycle, so this recycle is not necessary to the process's ability to
remove boron and fluoride. However, for full scale operation this recycle
is important to obtaining enhanced reaction kinetics and improvement in
the settling and concentrating characteristics of the precipitated solids.
Water with precipitating solids flows out of the first tank reactor into a
second reactor stage consisting of two mechanically agitated tanks
arranged for parallel flow, wherein the precipitation reactions proceed to
completion. The use of two reactors is optional. The residence time in
these tanks is preferably about 16 minutes. The number of tanks used may
vary based on hydraulic considerations.
Water and solids leaving the second reactor stage proceed to a clarifying
stage consisting of two clarifiers, arranged for parallel flow, with a
design overflow rate of 1.0 gpm/sq. ft. The use of two clarifiers is
optional. The number of clarifiers may vary depending upon hydraulic and
other process engineering considerations. The clarifiers are optionally
equipped with peripheral and, in case of large diameter units, radial
discharge launders, mechanical sludge rakes and sludge underflow pumps for
recirculation and disposal of precipitated solids.
Solids from the clarifier underflow are recirculated at a maximum rate of
about 8% of the volume of the feed water. Higher recirculation percentages
are not believed to be detrimental to the process. An amount of underflow
solids equal to the amount generated daily is directed to a sludge storage
tank for subsequent re-use in the gold plant recovery circuit.
In an optional operation when the water is to be discharged into a river,
and the discharge temperature is regulated, water from the clarifier
overflow is directed to cooling towers for cooling to within 2.degree. C.
of the ambient water temperature of the river into which it is to be
discharged.
As a result of the foregoing treatment, boron concentrations are reduced to
below, for example, 2 mg/L boron. In one embodiment, boron is reduced from
between about 0.8 mg/L and about 5 mg/L to less than about 0.7 mg/L, for
example, from above about 1 mg/L to below about 0.6 mg/L or, for example,
from above about 0.7 to below about 0.6 mg/L. At such low levels of boron
concentration, final boron levels to be achieved in the effluent are an
order of magnitude lower than stated to be achieved by prior chemical
precipitation methods referring to reduction from, for example, 10 mg/L to
5 mg/L. Similarly, fluoride ion concentration is reduced from at least
about 1.3 mg/L fluoride ions to less than about 0.9 mg/L fluoride ions. As
such, the process is effective for boron and fluoride ion removal in much
more difficult conditions and ranges than that achieved by prior
processes. The process also has proven efficacy for B and F removal in a
large scale operation involving the treatment of groundwater high in TDS
and containing nominal amounts of B and F of unknown speciation, and
further containing other naturally occurring species such as iron,
arsenic, chlorides, copper, manganese, zinc and others. Laboratory test
work was performed using actual groundwater and did not require spiking of
deionized water with known species of B and F.
In one preferred embodiment, sludge from the sludge storage tank is
directed to neutralization of pressure oxidized ore slurry prepared in
accordance with the processes disclosed in Thomas et al. U.S. Pat. No.
5,071,477, Thomas et al. U.S. Pat. No. 5,489,326, or Marchbank et al. U.S.
Pat. No. 5,536,297, the entire disclosures of which are expressly
incorporated herein by reference. To describe the use of the alkaline
sludge in more detail, it is useful to briefly describe major features of
the relevant portion of the autoclave circuit. Oxidized slurry leaving the
autoclave is passed to a series of flash tanks where steam is flashed off
to cool the slurry. Steam from each flash tank is recycled and contacted
with autoclave feed slurry in a complementary splash condenser, operated
at substantially the same pressure as the flash tank, for preheating the
feed slurry to the autoclave. Typically, between 1 and 3 flash tanks and
between 0 and 3 condensers are employed. Steam leaving each of flash tanks
is optionally passed through cyclones for recovery of entrained solids.
The recovered solids are blended back into the oxidized slurry. Oxidized
slurry from the autoclave flash tanks has a solids content of at least
about 30% by weight, preferably at least about 35% by weight, and contains
soluble sulfates, iron salts, and arsenates. The slurry is transferred to
an intermediate agitated storage tank.
In order to condition the slurry for gold recovery operations, the
temperature of the hot oxidized slurry is reduced to between about
90.degree. F. and about 140.degree. F., preferably between about
100.degree. F. and about 120.degree. F., by passing the slurry through a
series of shell and tube coolers. The temperature of the slurry is reduced
by exchanging heat from the slurry to a cooling water stream. Cooling
water is obtained from a recirculating system in which the water is
recycled through a crossflow, induced draft cooling tower. Cooled oxidized
slurry which discharged from the coolers is fed continuously through a
series of agitated neutralization tanks. In accordance with the process
described in Thomas et al. U.S. Pat. No. 5,071,477 and Thomas et al. U.S.
Pat. No. 5,489,326, the cooled oxidized slurry is preferably directly
neutralized without either washing the slurry or separating solids
therefrom prior to neutralization. By omitting any washing operation
between the autoclave and the neutralization operation, as is preferred
but not required, the volume of materials handled is reduced and the need
for other ancillary operations such as wash water recovery is avoided.
In the neutralization process the pressure oxidized gold slurry from the
autoclave circuit is neutralized with a base, normally a slurry of calcium
hydroxide is used, to raise its pH to between about 9 and about 11.5,
preferably between about 10 and about 11, preferably about 10 or 10.5,
according to the process described in Thomas et al. U.S. Pat. No.
5,071,477 and Thomas et al. U.S. Pat. No. 5,489,326, and to a pH of about
7 to 8.7 according to the Marchbank et al. U.S. Pat. No. 5,536,297.
In accordance with the present process for B and F removal, the alkaline
sludge from the water treatment process, with a solids concentration of
about 20 to 40% by weight, is metered into the first of three in series
neutralization tanks. The sludge consists of a substantial water fraction,
and a solids fraction consisting of about 94% by weight CaCO.sub.3 and 6%
magnesium compounds. The boron and fluoride content of the sludge, which
is on the order of about 0.5 g B per kg of sludge and about 0.8 g F per kg
of sludge, with a standard deviation of about (+/-) 0.1 g/kg for B and F,
has proven to be stable. The carbonate/hydroxide sludge partially
neutralizes the sulfuric acid contained in the oxidized gold slurry from
the autoclaves and raises the pH of the slurry from about 1.0 to a range
of 3.0 to 4.0. The reaction products are calcium sulfate, magnesium
sulfate and carbonic acid which decomposes to form carbon dioxide and
water. The carbon dioxide formed is released to the atmosphere. This step
is important to prevent the formation of calcium carbonate with the
addition of lime in the second stage of neutralization. In the second set
of neutralization reactors, the pH of the partially neutralized gold
slurry is then raised with the addition of a lime slurry to the desired pH
of about 10 to 10.5 for, for example, the process described in Thomas et
al. U.S. Pat. No. 5,071,477, and to a pH of about 7.0 to 8.7 for, for
example, the process described in Marchbank et al. U.S. Pat. No.
5,536,297. The third and final set of neutralization reactors serve as a
backup to the preceding tanks. Additional lime slurry is added to the
third neutralization tank, as necessary, to achieve the preferred pH
range. The following overall reaction equations describe the process:
CaCO.sub.3.MgCO.sub.3 +H.sub.2 SO.sub.4 .fwdarw.CaSO.sub.4 +MgSO.sub.4
+2H.sub.2 CO.sub.3 (1)
H.sub.2 CO.sub.3 .fwdarw.CO.sub.2 +H.sub.2 O (2)
Mg(OH).sub.2 +H.sub.2 SO.sub.4 .fwdarw.MgSO.sub.4 +2H.sub.2 O (3)
Ca(OH).sub.2 +H.sub.2 SO.sub.4 .fwdarw.CaSO.sub.4 +2H.sub.2 O (4)
CO.sub.2 +Ca(OH).sub.2 .fwdarw.CaCO.sub.3 +H.sub.2 O (5)
Compressed oxygen (or air) is optionally sparged into the oxidized gold
slurry in the neutralization tanks to convert ferrous iron to ferric iron,
as the former consumes cyanide in any subsequent cyanidation process. The
neutralized gold-bearing slurry, having a solids content of 30 to 40% by
weight and a temperature of about 25 to 35.degree. C., is then pumped at a
controlled rate to the gold recovery operation.
The gold may be recovered from the oxidized slurry by any of a number of
means presently known or hereafter developed. In one preferred embodiment,
the gold in the oxidized slurry is recovered by a conventional
carbon-in-leach (C-I-L) cyanidation or cyanidation followed by
carbon-in-pulp (C-I-P) in which the neutralized slurry is passed to a
series of agitated carbon-in-leach tanks countercurrently to a flow of
granular activated carbon. Loaded carbon recovered from the
carbon-in-leach operation is stripped with hot alkaline cyanide solution
and gold is recovered from the stripping solution by conventional means
such as electrowinning and refining.
The following examples illustrate aspects of the current invention.
EXAMPLE 1
Raw well water from Barrick Gold Corporation's Meikle Mine (See Table 1 for
analysis) was treated in accordance with the method of the invention, with
treatment goal target limits of 0.61 mg/L for boron and 0.87 mg/L for
fluoride. Water (750 ml) was placed into each of eight one-liter jars
(J3-1 through J3-8), which were in turn placed in a water bath at
100.degree. F. (38.degree. C.) and mixed using a paddle stirrer at 30 rpm.
Magnesium (20 mg/L) as a solution of magnesium sulfate (20 g/L MgSO.sub.4)
was added to the first four jars (J3-1 through J3-4), and magnesium (80
mg/L) as a solution of magnesium sulfate (20 g/L MgSO.sub.4) was added to
the second four jars (J3-5 through J3-8). Sufficient lime as a 25 g/L
Ca(OH).sub.2 slurry was added to each jar to raise the pH to the selected
target pH values. After reaching the desired pH, mixing of each jar was
continued for 30 minutes. The contents of each jar were filtered using
0.45 micron filter paper. A 250 ml fraction of the filtrate of each water
sample (for each jar) was analyzed for several analytes before
neutralization. A second 250 mL fraction of each water sample was then
neutralized by titration with concentrated sulfuric acid, and the
conductivity, total dissolved solids (TDS), and other analytes were
determined. Each filter/residue sample was weighed. The boron content of
several residue samples was determined by ICPAES (inductively coupled
plasma atomic emission spectroscopy), after dissolving in concentrated
nitric acid. The fluoride content of several residue samples was
determined by the ion specific electrode method, after dissolving in
concentrated nitric acid.
Test results for J3-1 to 3-8 are summarized in Table 4 and are illustrated
graphically in FIGS. 2 and 3. Graphs are presented for the amount of boron
and fluoride remaining in the test samples versus the amount of magnesium
that was precipitated at the various pH values of each jar. Based on these
results, it is evident that for tests J3-1 to J3-3, with the pH ranging
from 9.43 to 10.21, very little magnesium precipitated even though over
40% of the silicon was precipitated, virtually no boron was removed, and
insufficient fluoride was removed to meet the treatment target limit of
0.87 mg/L F max.
In test J3-4, the residual boron concentration was 0.488 mg/L and the
fluoride concentration was 0.792 mg/L--both concentrations being well
below the respective treatment goal target limits of 0.61 mg/L for boron
and 0.87 mg/L for fluoride.
The low concentrations of boron and fluoride in jar J3-4 were achieved with
the precipitation of 26.2 mg/l of Mg (60.4%) and the reduction of silicon
from 21.7 mg/l to 9.0 mg/L. Also in jar J3-4, with a final treatment pH of
10.4, the TDS before neutralization was reduced from 566
mg/l to 373 mg/l (based on electrical conductivity measurement) or by
34.1%. After neutralization with sulfuric acid to a pH of 7.49, the TDS
concentration was 382 mg/L. The TDS concentrations before and after
neutralization are reasonably close to the treatment goal target limit of
358 mg/L, considering the results by electrical conductivity are not very
accurate at high concentrations and could be lower had the analytical
weight method been used for the analysis.
TABLE 4
__________________________________________________________________________
Ca(OH).sub.2
pH H.sub.2 SO.sub.4
Neut'd
(AN)
(AN)
Sample
Added
H TDSfter
Conduct.
Mg
Si
added
pH
TDS
Conduct.
ID s.u.
30 min.
mg/l
umhos/cm
mg/l
mg/l
mg/l
mg/l
mg/l
s.u.
mg/l
umhos/cm
__________________________________________________________________________
Blank 8.08 1.44
.903
23.4
21.7
J3-1
244.4
9.47
4403
662
.848
40.8
8.9
81.2
7.58
700
J3-2
311.1
9.85
4265
638
.843
39.6
7.8
135.4
7.57
688
J3-3
377.7
10.24
10.21
418
630
.837
36.3
8.4
145.2
7.51
672
J3-4
466.7
10.75
10.4
373
562
.4882
17.2
9.0
100.9
7.49
578
Blank
.08
21.7 127
J3-5
222.3
9.22
9.21
696
1044
.851
127
21.7
150.2
7.52
1112
J3-6
288.9
9.61
9.62
676
1014
.849
120
21.5
115.7
7.46
1084
J3-7
377.7
10.10
9.97
654
981
.7419
114
15.8
107.8
7.47
1039
J3-8
688.9
10.54
10.49
584
877
.0876
24.6
1.1
49.2
4.6
859
__________________________________________________________________________
Notes
1/ This test was done with 750 ml samples of Meikle Mine well water at
100.degree. F.
2/ TDS were measured by electrical conductivity meter.
3/ Results for the blank are those from Jar Test #1.
4/ (AN) = after neutralization
In jars J3-5 to 3-8, with the high dosage of 80 mg/L Mg, the results were
similar to those with 20 mg/L Mg dosage except that the reductions in
boron and fluoride were significantly higher at the higher Mg dosage.
In jar J3-8, with a pH of 10.49 and before neutralization, the boron
concentration was reduced to 0.087 mg/L and fluoride was reduced to 0.066
mg/L. Also, Mg was reduced from 127 mg/L in the blank to 24.6 mg/L and Si
from 21.7 mg/L in the blank to 1.1 mg/L in the treated sample.
On the basis of the foregoing, therefore, it is concluded that the addition
of magnesium to the feed water at a high pH will reduce the concentrations
of boron and fluoride to the desired target limits. In particular, in
comparing results for jars J3-5, 6 and 7 with those of jars J3-8, it is
evident very little boron and insufficient amounts of fluoride are removed
with no precipitation, or very little precipitation of Mg and Si. With
magnesium sulfate as the source of magnesium for coagulation, care must be
taken in selecting a Mg dosage for the removal of boron and fluoride which
is high enough to reach the target boron and fluoride levels but low
enough in order to also meet the TDS target limit.
EXAMPLE 2
Meikle Mine well water (750 ml) was added to jars J11-1 to J11-4 and
preheated to 100.degree. F. (38.degree. F.) by immersion in a water bath.
Magnesium Sulfate (20 g/L) as a solution of magnesium (25 mg/L MgSO.sub.4
concentration) was added to each jar while mixing at 30 rpm and holding
the temperature constant at 100.degree. F. (38.degree. F.). The pH of each
jar was raised by adding sufficient hydrated lime slurry containing 25 g/L
Ca(OH).sub.2. Mixing was continued at 30 rpm for 30 minutes and, after
mixing, the pH of each jar was recorded. Test samples were filtered and
analyzed for several analytes, then neutralized and the acid consumption
was determined. The dried filter residue was weighed to determine the
amount of solids produced. The tests were repeated with jars J11-5 to
J11-8 using 30 mg/L Mg. The test results are summarized in Tables 5 to 7
and illustrated graphically in FIGS. 4 and 5.
Raw water quality, as represented by the results for the blank in Table 6,
is fairly representative of the Meikle well water quality encountered
during the testing. The original pH of the water was 7.81 and the TDS was
about 512 mg/L as measured by electrical conductivity. The actual
concentrations of TDS vary somewhat from the values given since
measurement of TDS by conductivity is inferred and varies with pH.
Discrepancies between the two methods of measurement normally increase at
high pH.
With 20 mg/L Mg, the initial boron concentration of 0.986 mg/L was reduced
to 0.60 mg/L before neutralization of the treated sample in jar J11-4 at
pH 10.55.
With a higher magnesium dosage of 30 mg/L, equivalent boron reductions to
those obtained with 20 mg/L Mg dose were achieved at lower pH values. In
jar J11-7, with a pH of 10.27 after treatment, the boron was reduced to
0.656 mg/L before neutralization. In jar J11-8 with a pH of 10.48, the
residual boron concentration was 0.45 mg/L before neutralization. One
especially preferred process therefore involves a magnesium dose of about
30 mg/L and an operating pH of about 10.3. In another preferred process
with a slightly higher pH of about 10.5, the boron concentration is also
well below the objective target limit. With a 20 mg/L Mg dose, the
fluoride residual in jar J11-2 at pH of 10.27 was 0.85 mg/L, in jar J11-3
at a pH of 10.40 it was 0.77 mg/L, and in jar J11-4 at a pH of 10.55 it
was 0.67 mg/L. All fluoride concentrations in these jars were below the
objective target limit of 0.87 mg/L. It is therefore discovered that
fluoride can be reduced to the target limit over a pH range of about 10.2
to about 10.6, and within this range the operating pH can be predicated
upon the requirements for the removal of boron and TDS.
With the 30 mg/L dose, the fluoride target limit was achieved in all jars,
J11-5 to J11-8. In jar J11-7 with a pH of 10.27, the residual fluoride
concentration was 0.66 mg/L and in jar J11-8 with a pH of 10.48, it was
0.52 mg/L.
Based on the target boron and fluoride reductions, especially preferred
processes embody the test conditions for jars J11-4, J11-7 and J11-8. In
jar J11-4 with a pH of 10.55, the TDS before neutralization was reduced to
363 mg/L, which is acceptable in view of the target limit of 358 mg/L. In
jars J11-7 and J11-8, with treatment pH values of 10.27 and 10.48
respectively, the respective TDS concentrations before neutralization were
397 and 375 mg/L. These concentrations are above the target limit of 358
mg/L and seem to be influenced by the higher amount of salt added with the
higher magnesium dose. There was no significant impact on the TDS
concentrations after neutralization to pH 7.5.
TABLE 5
__________________________________________________________________________
Mg 2/ Ca(OH).sub.2
pH pH After
(1) (1) (2) (2) (3) (3)
Sample
Added
Target
added
After
30 min.
pH
H.sub.2 SO.sub.4
pH
H.sub.2 SO.sub.4
pH
H.sub.2 SO.sub.4
ID mg/l
pH
mg/l
Ca(OH).sub.2
test
Actual
mg/l
Actual
mg/l
Actual
mg/l
__________________________________________________________________________
Blank 7.81*
J11-1
20
10.00
290.0
9.98
8.51
73.9
8.00
84.4
7.53
94.92
J11-2
20
10.20
333.3
10.23
10.27
6.85
106.1
J11-3
20
10.40
366.7
10.47
10.40
8.47
70.9
8.00
77.4
7.49
90.3
J11-4
20
10.60
400.0
10.65
10.55
8.52
54.2
7.96
61.8
7.50
66.7
Blank
J11-5
30
10.60
9.98.3
8.51
70.0
8.01
80.4
7.46
87.4
J11-6
30
10.20
333.3
8.520
71.9
8.02
77.4
7.51
82.3
J11-7
30
10.20
366.7
8.467
61.0
8.00
63.2
7.48
69.4
J11-8
30
10.60
420.0
10.52
10.48
8.52
46.4
7.99
50.6
7.52
53.0
__________________________________________________________________________
*original pH of sample.
Notes
1/ The test was done with 750 ml samples of Meikle Mine well water at
100.degree. F.
2/ Added as Magnesium Sulfate.
3/ (1) = Target pH 8.5; (2) = Target pH 8.0; (3) Target pH 7.5
TABLE 6
__________________________________________________________________________
pH After (AN)
(AN)
Sample
30 min.
TDS
Conduct.
F
TDS
Si
Mn
Fe
ID test
umhos/cm
mg/l
mg/l
umhos/cm
mg/l
mg/l
mg/l
mg/l
mg/l
mg/l
__________________________________________________________________________
Blank
7.81*
512
768 1.31 0.986
25.0
22.9
95.5
<0.01
0.39
J11-1
608
33.1
19.5
6.6
<0.01
<0.07
J11-2
10.27
592
28.8
17.5
7.3
<0.01
<0.07
J11-3
10.40
562
20.7
13.3
8.0
<0.01
<0.07
J11-4
10.55
545
13.7
11.2
9.6
<0.01
<0.07
Blank
J11-5
657
42.1
20.2
8.1
<0.01
<0.07
J11-6
10.20
632
35.5
16.0
8.6
<0.01
<0.07
J11-7
10.27
596
27.0
12.4
9.2
<0.01
<0.07
J11-8
10.48
563
14.5
8.6
12.6
<0.01
<0.07
__________________________________________________________________________
*originai pH ot sample
Note
1/ TDS was measured by electrical conductivity
2/ (AN) = after neutralization
TABLE 7
__________________________________________________________________________
Mg 1/
Ca(0H).sub.2
pH After H.sub.2 S0.sub.4
Neut'd
(AN)
(AN)
Sample
Added
Added
30 min.
TDS
Conduct.
F
Mg
Added
pH
TDS
Conduct.
ID mg/l
mg/l
test
mg/l
umhos/cm
mg/l
mg/l
mg/l
mg/l
mg/l
s.u.
mg/l
umhos/cm
__________________________________________________________________________
Blank 512
768 1.31
.986
25.0
22.9
J11-1
290.0
9.97
406
608
0.91
.824
33.1
19.5
94.92
7.53
432
650
J11-2
333.3
10.27
395
592
0.85
.814
28.8
17.5
106.1
6.85
420
628
J11-3
366.7
10.40
374
562
0.77
.729
20.7
13.3
90.3
7.49
381
572
J11-4
400.0
10.55
363
545
0.67
.600
13.7
11.2
66.7
7.50
358
537
Blank
J11-5
293.3
9.96
438
657
0.86
.804
42.1
20.2.
87.4
7.46
464
697
J11-6
30
333.3
10.20
422
632
0.79
.769
35.5
16.0
82.3
7.51
439
659
J11-7
366.7
10.27
397
596
0.66
.656
27.0
12.4
69.4
7.48
404
607
J11-8
420.0
10.48
375
563
0.52
.450
14.5
8.6
53.0
7.52
370
555
__________________________________________________________________________
Notes
1/ Mg added as Magnesium Sulfate.
2/ (AN) = after neutralization
In jars J11-4 and J11-8, there was a decrease from the values obtained
before neutralization and in jar J11-7, the concentration increased by 7
mg/L to 404 mg/L.
The lime [Ca(OH).sub.2 ] consumption in jar J11-4 to achieve a treatment pH
of 10.55 was 400 mg/L or 3.34 lbs Ca(OH).sub.2 per 1000 gallons of water
to be treated. In jar J11-7, 366.7 mg/L (3.06 lbs/1000 gallons) of lime
was required to raise the pH to 10.27, and in jar J11-8, 420 mg/L (3.50
lbs/1000 gallons) of lime was required to raise the pH to 10.48.
The amount of sulfuric acid consumed to neutralize treated water samples to
pH of about 7.5 was 66.7 mg/L (0.56 lb/1000 gallons) in jar J11-4, 69.4
mg/L (0.58 lb/1000 gallons) in jar J11-7, and 53.0 mg/L (0.44 lb/1000
gallons) in jar J11-8. The acid amounts required for target pH values of
8.0 and 8.5 are presented in Table 5.
On the basis of the foregoing, it is concluded that treatment goals for
TDS, fluoride and boron can be reached by lime softening with the addition
of 20 mg/L Mg at a pH of about 10.6 as indicated by the results of jar
test J11-4.
It is evident that for the same magnesium dosage the higher pH results in a
greater reduction of fluoride and boron. The preferred maximum pH is 10.6,
in this application, to limit the degree of softening to the level where
treatment target levels are achieved. The degree of removal of these
elements appears to be a function of the amounts of magnesium and silica
that are precipitated in the reaction.
EXAMPLE 3
The procedure of Example 2 was performed using jars J12-1 to J12-4, except
the dosage was 15 mg/L of magnesium as a solution of magnesium sulfate
(concentration 25 g/L MgSO.sub.4) added to each jar. The tests were
repeated with jars J12-5 to J12-8 using 25 mg/L Mg as the magnesium
dosage.
Test results are presented in Tables 8 to 10. Residual boron and fluoride
concentrations after the tests are plotted against pH in FIGS. 6 and 7.
The initial pH was 7.81, the TDS concentration was 512 mg/L, the fluoride
concentration was 1.31 mg/L, and the boron concentration was 0.986 mg/L.
The target limit for boron of 0.61 mg/L was achieved with a 15 mg/L Mg
dosage in jar J12-3 at a pH of 10.54 and in jar J12-4 at a pH of 10.74;
the residual boron concentrations being 0.547 mg/L and 0.434 mg/L before
neutralization for the two respective jars.
With the higher dosage of 25 mg/L Mg (J12-5 to J12-8), similar boron
reductions were achieved as for jars J12-3 and J12-4 but at a lower
treatment pH. For example, in jar J12-7, the pH was 10.37 and boron
residual concentrations were 0.489 mg/L before neutralization. In jar
J12-8, the pH was 10.48 and boron residual concentration was 0.3 mg/L.
The fluoride target limit of 0.87 mg/L was achieved with a 15 mg/L Mg
dosage in jars J12-2 at pH 10.43, J12-3 at pH 10.54, and J12-4 at 10.74,
final concentrations being 0.89 mg/L, 0.77 mg/L and 0.69 mg/L,
respectively, before neutralization. With a magnesium dosage of 25 mg/L,
the fluoride target limit was achieved in jars J12-6 at pH 10.28, J12-7 at
pH 10.37, and J12-8 at pH 10.48. The respective residual fluoride
concentrations in jars J12-6 to J12-8 were 0.82 mg/L, 0.65 mg/L and 0.54
mg/L. These test results indicate that at equal treatment pH values more
fluoride can be removed with the higher magnesium dosage.
With regard to TDS, the test results of jars J12-3 and J12-4 show that the
treatment goal target limit of 358 mg/L can be met with the lower
magnesium dosage of 15 mg/L at operating pH values of 10.54 or 10.74 in
the two respective jars. The actual TDS test values recorded were 338 mg/L
for jar J12-3 and 336 mg/L for jar J12-4. At the higher magnesium dosage
the target limit for TDS was not achieved. It appears that at pH 10.25 and
over all of the original calcium carbonate alkalinity is precipitated and
any further increases in pH will add excess calcium hydroxide alkalinity
to the water. Also, with the higher magnesium dosage the added sulfate
becomes significant and impacts on the total dissolved solids content of
the water.
The lime consumption in jar J12-3 with a 15 mg/L Mg dose was 366.7 mg/L
(3.06 lbs/1000 gallons) to achieve a treatment pH of 10.54 and the acid
consumption for neutralization to pH 7.29 was 69.9 mg/L (0.58 lb/1000
gallons). For a dose of 25 mg/L Mg in jar J12-7, the lime consumption was
the same at 366.7 mg/L (3.06 lbs/1000 gallons) to achieve a treatment pH
of 10.37 and the acid consumption for neutralization to pH 7.49 was 68.8
mg/L (0.57 lb/1000 gallons).
TABLE 8
__________________________________________________________________________
Mg 2/
Ca(OH).sub.2
pH After
(1) (1) (2) (2) (3) (3)
Sample
Added
Added
pH After
30 min.
H.sub.2 S0
pH
H.sub.2 S0.sub.4
pH
H.sub.2 S0.sub.4
ID mg/l
mg/l
Ca(OH).sub.2
test
Actual
mg/l
Actual
mg/l
Actual
mg/l
__________________________________________________________________________
Blank
J12-1
15
290.0
10.25
10.25
8.50
81.7
8.00
88.5
7.49
95.2
J12-2
15
331.7
10.55
10.43
8.50
75.9
8.01
76.7
7.49
85.4
J12-3
15
366.7
10.76
10.54
8.50
54.8
7.96
62.5
7.29
69.9
J12-4
15
403.3
10.88
10.74
8.43
49.6
7.93
52.5
7.42
55.9
Blank
J12-5
25
290.0
10.21
10.20
8.52
83.5
8.02
91.4
7.50
100.2
J12-6
25
330.0
10.49
10.28
8.49
63.9
8.04
66.8
7.51
74.7
J12-7
25
366.7
10.68
10.37
8.52
49.1
7.99
63.9
7.49
68.8
J12-8
25
427.3
10.80
10.48
8.51
35.4
7.91
39.0
7.51
43.9
__________________________________________________________________________
Notes
1/ The test was done with 750 ml samples of Meikle Mine well water atn
100.degree. F.
2/ Mg added as Magnesium Sulfate.
3/ (1) = Target pH 8.5; (2) = Target pH 8.0; (3) = Target pH 7.5
TABLE 9
__________________________________________________________________________
(AN)
(AN)
Sample
pH After
Conduct.
F
TDS
Conduct.
B Ca
Fe
ID umhos/cm
mg/l
mg/l
umhos/cm
mg/ll
mg/l
mg/l
mg/l
mg/l
__________________________________________________________________________
Blank
7.81*
512
768 1.31 0.986
25.0
22.9
95.5
J12-1
10.25
577
0.95
413
619
27.81
19.8
7.59
0.18
0.09
J12-2
10.43
539
0.89
376
565
18.92
15.4
7.98
0.18
0.091
J12-3
10.54
508
0.77
342
513
12.3
8.00
0.18
0.095
J12-4
10.74
504
0.69
315
473
10.6
7.28
0.18
0.096
Blank
J12-5
10.20
623
0.89
442
662
36.05
19.0
7.75
0.1
<0.14
J12-6
10.28
586
0.82
408
612
27.41
15.0
8.6
<0.14
J12-7
10.37
550
0.65
371
556
16.689
11.2
9.3
<0.14
J12-8
10.48
525
0.54
342
511
8.34
<0.14
__________________________________________________________________________
*0riginal sample water.
Notes
1/ TDS was measured by electrical conductivity.
2/ (AN) = after neutralization
TABLE 10
__________________________________________________________________________
Mg 1/
Ca(OH).sub.2
pH After H.sub.2 SO.sub.4
Neut'd
(AN)
(AN)
Sample
Added
Added
30 min.
TDS
Conduct.
Added
pH
TDS
Conduct.
ID mg/l
mg/l
test
mg/l
umhos/cm
mg/l
mg/l
mg/l
mg/l
mg/l
s.u.
mg/l
umhos/cm
__________________________________________________________________________
Blank
512
768 1.31
.986
25.0
22.9
J12-1
15
290
577
.831
27.8
19.8
95.2
7.49
413
619
J12-2
15
331.7
10.43
539
.752
18.9
15.4
85.4.
7.49
376
565
J12-3
15
366.7
10.54
508
.547
9.5
12.3
69.9
7.29
342
513
J12-4
15
403.3
10.74
504
.434
3.0
10.6
55.9
7.42
315
473
Blank
J12-5
25
290.0
10.20
623
.775
36.0
19.0
100.2
7.50
442
662
J12-6
25
330.0
10.28
586
.701
27.4
15.0
74.7
7.51
408
612
J12-7
25
366.7
10.37
550
.489
16.6
11.2
68.8
7.49
371
556
J12-8
25
427.3
10.48
525
.354
7.38
8.34
43.9
7.51
342
511
__________________________________________________________________________
Notes
1/ Mg added as Magnesiurn Sulfate (MgSO.sub.4{l ).
2/ (AN) = after neutralization
Based on the foregoing, it is concluded that treatment goals for TDS,
fluoride and boron can be reached by lime softening with the addition of
15 mg/L Mg at a pH of 10.54 as indicated by the results of jar test J12-3.
Also, at equal pH values, the higher magnesium dosage will result in
higher reductions of boron and fluoride. The conclusion drawn in Example 2
that the degree of removal of boron and fluoride is a function of the
amounts of magnesium and silica that are precipitated in the reaction is
reaffirmed by the results of the Example 3.
In the interest of meeting the treatment goals and minimizing chemical
consumption, the pH of about 10.55 appears to be most preferred.
EXAMPLE 4
Meikle Mine well water (750 ml) was added to jars J13-1 to J13-4 and
preheated to 100.degree. F. (38.degree. F.) by immersion in a water bath.
Magnesium (20 mg/L Mg) as a solution of magnesium sulfate (25 g/L
MgSO.sub.4 concentration) was added to each jar while mixing at 30 rpm and
holding the temperature constant at 100.degree. F. (38.degree. F.). The pH
of each jar was raised to 10.75 by adding sufficient hydrated lime slurry
containing 25 g/L Ca(OH).sub.2. The solutions in jars J13-1 to J13-4 were
mixed at 30 rpm for 5, 10, 15 and 20 minutes, respectively, while
maintaining the temperature constant at 100.degree. F. (38.degree. F.).
The tests were repeated with jars J13-5 to J13-8 being mixed for 11, 18,
23 and 26 minutes, respectively.
The test results are summarized in Table 11. The results for jar J13-4
before neutralization appeared to be in error and were not used in the
evaluation. Instead, the results obtained for after neutralization were
used. The boron and fluoride concentrations after the test are plotted
against mixing time in FIGS. 8 and 9, and magnesium and fluoride
concentrations are plotted against mixing time in FIGS. 10 and 11.
Target limits for boron and fluoride were achieved with 5 minutes of
mixing, the shortest time used in the test. Fluoride, however, continued
to decrease with mixing time and appeared to be stable after 15 to 18
minutes of mixing (jars J13-3 and J13-6). Boron also continued to decrease
with mixing time in jars J13-1 to J13-4 and was lowest in jar J13-4 after
20 minutes of mixing. In jars J13-5 to J13-8, the lowest boron
concentration was achieved in jar J13-5 after 11 minutes of mixing. Longer
mixing times, such as for jars J13-7 and J13-8, had no further impact on
the reduction of boron.
The concentrations of magnesium and silicon continued to decrease with
time. In jars J13-1 to J13-4, the lowest concentrations of magnesium and
silicon were achieved after 20 minutes of mixing, remaining concentrations
being 2.9 and 9.5 mg/L, respectively. In jars J13-5 to J13-8, the lowest
concentration of magnesium of 2.4 mg/L was achieved in jar J13-6 after 18
minutes of mixing. However, this minimum magnesium concentration may be
due to the high pH of 10.88 of the four jars rather than the mixing time.
The silicon, however, continued to decrease and was lowest even after 26
minutes of mixing, albeit, the rate of decrease was substantially lower
after the initial 5 minutes of mixing.
TABLE 11
__________________________________________________________________________
Mg 3/
Mixing
Ca(OH).sub.2
pH H.sub.2 SO.sub.4
Neut'd
(AN)
(AN)
Sample
Added
TDSer
Conduct.
Added
Conduct.
ID mg/l
mg/1st
umhos/cm
mg/ll
mg/l
Mg/l
mg/l
umhos/cm
__________________________________________________________________________
Blank
J13-1
.4827
6.3
84.7
511
J13-2
.4643
5.0
78.3
493
J13-3
.4421
4.0
70.9
483
J13-4
.0091
0.5
7.50
476
Blank
.8464
23.7
19.8
J13-5
.3851
4.0
69.9
492
J13-6
.3929
2.4
84.1
494
J13-7
.4287
3.6
75.8
485
J13-8
.4467
3.86
72.1
__________________________________________________________________________
483
Notes
1/ The test was done with 750 ml samples of Meikle Mine well water at
100.degree. F.
2/ TDS were measured by electrical conductivity meter.
3/ Mg added as Magnesium Sulfate.
4/ (AN) = after neutralization
5/ The results for B, Mg, and Si removal for test J134 in this Table
appear to be in error as noted above.
Although boron and fluoride treatment goal limits were reached after only 5
minutes of mixing, the optimum reaction time must be based on the
softening reaction for the precipitation of magnesium, calcium and
silicon. Based on the results of this test, the preferred reaction time in
the external reactor appears to be about 15 to 20 minutes. Selection of
the optimum time must also be based on economic considerations, since some
post-precipitation of the lime softening reaction products, which should
be minor, can be allowed to occur in the clarifiers.
EXAMPLE 5
Meikle Mine well water (750 ml) was added to jars J14-1 to J14-4 and
preheated to 100.degree. F. (38.degree. F.) by immersion in a water bath.
Magnesium (13, 14, 16 and 17 mg/L Mg) as a solution of magnesium sulfate
(25 g/L concentration MgSO.sub.4) was added to each jar while mixing at 30
rpm and holding the temperature constant at 100.degree. F. (38.degree.
F.). The pH of each jar was raised to 10.75 by adding sufficient hydrated
lime slurry containing 25 g/L Ca(OH).sub.2. The solutions in jars J14-1 to
J14-4 were mixed at 30 rpm for 18 minutes, respectively, while maintaining
the temperature constant at 100.degree. F. (38.degree. F.).
Test results for this example are summarized in Table 12. The boron and
fluoride concentrations after the test are plotted against the amount of
magnesium added in FIG. 12.
The actual pH in the four jars after the test was 10.67 in jar J14-1, 10.68
in jar 14-2, 10.70 in jar J14-3 and 10.71 in jar J14-4. The lowest boron
concentration of 0.396 mg/L was achieved in jar J14-3 with a magnesium
dosage of 16 mg/L. For fluoride, the lowest concentration was achieved in
jar J14-4 with a magnesium dosage of 17 mg/L.
Silicon continued to decrease with increasing magnesium dosage. It is also
noted that the TDS concentrations all exceeded the treatment goal limit of
358 mg/L, possibly because of the high pH resulting in the presence of an
excess of calcium hydroxide alkalinity.
On the basis of these test results, it is concluded that based on boron
removal, the preferred magnesium dosage is 16 mg/L. Based on fluoride
removal, the preferred dosage is 17 mg/L with a treatment pH of about
10.70. Treatment goal limits for boron and fluoride were achieved with the
minimum magnesium dose of 13 mg/L and a treatment pH of 10.67.
The reduction of silicon depends upon the amount of magnesium that is
precipitated; hence, the lowest concentration is achieved at the highest
magnesium dosage.
TABLE 12
__________________________________________________________________________
Mg 3/
Mixing
Ca(OH).sub.2
pH After
Sample
Added
Time
Added
TDSin.
Conduct.
F Si
ID mg/l
min.
mg/l
umhos/cm
mg/l
mg/l
mg/l
mg/l
__________________________________________________________________________
Blank
J14-1
13
18
433
10.67
440
660
.66
.523
3.0
11.3
J14-2
14
18
436.7
10.68
426
641
.64
.492
2.8
11.2
J14-3
16
18
400
10.70
438
660
.62
.396
2.5
10.8
J14-4
17
18
440
10.71
444
666
.60
.420
2.6
10.4
__________________________________________________________________________
Notes
1/ The test was done with 750 ml samples of Meikle Mine well water at
100.degree. F.
2/ TDS were measured by electrical conductivity meter.
3/ Mg added as Magnesium Sulfate.
EXAMPLE 6
Meikle Mine well water (750 ml) was added to jar J15-1 at ambient
temperature of 23.1.degree. C. (73.6.degree. F.) with no preheating.
Magnesium (16 mg/L Mg) as a solution of magnesium sulfate (25 g/L
MgSO.sub.4 concentration) was added to each jar while mixing at 30 rpm and
holding the temperature constant at ambient. The pH of each jar was raised
to 10.75 by adding sufficient hydrated lime slurry containing 25 g/L
Ca(OH).sub.2. The solution was mixed at 30 rpm for 18 minutes.
Test results are summarized in Table 13. The pH after the test was 10.79
and boron and fluoride concentrations were below the established target
limits for these parameters. Specifically, the final boron concentration
was 0.514 mg/L and the fluoride concentration was 0.71 mg/L. The TDS
concentration was 447 mg/L (by electrical conductivity measurement), which
is in excess of the target limit of 358 mg/L. In comparing these results
with those for jar J14-3 of the previous test, it will be noticed that the
results for jar J15-1 are all higher including those for magnesium,
silicon and calcium. The reason for this is not entirely clear since
higher concentrations may be due to slightly higher solubilities at the
lower temperature for these elements rather than because of a slower
reaction time.
At ambient temperature, a longer mix time is desirable to minimize
post-precipitation in the clarifier. The removal efficiencies for boron
and fluoride are slower at the ambient temperature of 73.6.degree. F.
compared to those at 100.degree. F. Boron and fluoride removals are
sufficient to meet target limits at ambient
temperature, hence operating at a much reduced temperature from the normal
feed water temperature of 100.degree. F. during the winter months should
not significantly impact on the reductions of boron and fluoride.
TABLE 13
__________________________________________________________________________
Mg 3/
Mixing
Ca(OH).sub.2
pH After
Sample
Added
Time
Added
18 min.
TDS
Conduct.
F B
Si
ID mg/l
min.
mg/l
test
mg/l
umhos/cm
mg/l
mg/l
mg/l
mg/l
__________________________________________________________________________
Blank l
J15-1
16
18
10.79
447
671
.71
.514
12.8
13.2
__________________________________________________________________________
Notes
1/ The test was done with a 750 ml sample of Meikle Mine well water at
73.6.degree. F.
2/ TDS were measured by electrical conductivity meter.
3/ Mg added as Magnesium Sulfate.
EXAMPLE 7
Barrick Goldstrike Mines Inc. (Barrick) owns and operates the Goldstrike
Mine located approximately 27 miles north of Carlin, Nev. Part of the
mining operation involves the control of groundwater by pumping from a
network of wells located peripheral to the ore deposits. The water is used
in the mining operations as well as for irrigation purposes. The quality
and quantity of the discharge of excess groundwater to the Humboldt River
is regulated by the Nevada Division of Environmental Protection (NDEP).
The discharge limits require reduction of concentrations of total
dissolved solids (TDS), fluoride and boron and to lower the temperature of
the groundwater prior to discharge. The TDS, fluoride and boron
concentrations in the groundwater average about 600 mg/L, 1.4 mg/L and 0.8
mg/L, respectively, and the effluent limits for discharge are 425 mg/L,
1.0 mg/L, and 0.75 mg/L, respectively. The temperature of the groundwater
is in the range of 140.degree. F. at the discharge of the pumping wells.
The permit requires that the discharge meets the temperature of the
Humboldt River within .+-.2.degree. C. The permit also limits the
concentration of unionized ammonia nitrogen in the discharge to 0.02 mg/L;
the concentration in the raw water varies between 0.80 to 1.20 mg/L
ammonia as nitrogen. The maximum permissible rate of discharge is limited
to 70,000 gpm (all references to gpm herein are to U.S. gpm).
The treatment process is illustrated in FIG. 1A, and includes raw water
storage in a surge pond 10, raw water pumping via a 48 inch diameter
pipeline 11 to the treatment facility, lime softening and the reduction of
boron and fluoride in three external solids contact reactors 13, 14 and
15, two 210-foot diameter clarifiers 17, 18, sludge pumps 19, 20,
re-carbonation and neutralization of the clarified water in an energy
dispersion tank 23, cooling in two forced draft countercurrent flow
cooling towers 25, 26 each containing ten cells, and clarifier sludge
storage tank 28 and loading and unloading facilities for truck transport
and reuse of the sludge in the mine's autoclave circuit. The pH of the
final cooling tower discharge is adjusted to 7.5 or slightly lower to meet
the permit limit for unionized ammonia nitrogen.
Chemical systems include storage, make-up and feed facilities for
quicklime, anhydrous magnesium sulfate, an anionic polyelectrolyte, and
sulfuric acid. An antiscalent chemical is added to the intake of the raw
water pumps to minimize scaling in the pumps and pipeline to the treatment
plant.
To enhance the reduction of boron and fluoride, the raw water is
preconditioned with magnesium sulfate 1 prior to the water reaching the
solids reaction tank, 13. In tank 13, the pH is raised to about 10.3 using
a slurry of lime 2. Clarifier underflow solids 3 are recycled to the
solids reaction tank 13 to act as a catalyst in the precipitation
reactions and to produce larger calcium carbonate crystals which will
settle faster in the clarifiers and produce a denser sludge. Lime slurry 2
is added to the external reaction tanks 14 and 15, as necessary, to
maintain the target pH. The retention time in tank 13 is about 9.0 minutes
based on the design flow rate of 65,000 gpm. Precipitation reactions are
completed in the external reaction tanks, 14 and 15. The retention time of
each external reaction tank is about 17.0 minutes at one-half of the
design flow rate.
Slurry discharging from the external reaction tanks 14 and 15 is conveyed
to two 210-foot diameter clarifiers, operating in parallel, for the
removal of suspended solids. A flocculent 4 is added to the clarifier
influent to increase the efficiency of liquid/solids separation. The
clarifiers are sized for a net overflow rate of 1.0 gpm/sq. ft. based on
the design flow rate and are equipped with a center pier bridge support, a
50-foot diameter center feed well, radial overflow launders in addition to
the peripheral launder, and a rotating sludge rake complete with a
hydraulic drive plus power lifting device.
The clarifier overflow discharges to an energy dispersion tank where dilute
sulfuric acid 5 is added for re-carbonation and neutralization of the
clarifier overflow water to a pH of about 8.3. The water is then treated
in two banks of cooling towers for temperature reduction.
Two banks of induced draft cooling towers are provided, each containing ten
cells equipped with variable speed fans. The cell dimensions are 54-foot
wide by 54-foot long by 49-foot high. The interior fill material consists
of grid fill to a depth of about 18 feet. The two banks of cooling towers
operate in series. At the end of the second cooling tower basin, the
cooled water overflows a weir and discharges into the water conveyance
pipeline intake sump. The final pH of the discharge is adjusted to about
7.5 by adding a controlled amount of dilute sulfuric acid 5 through a pipe
diffuser located in the cooling tower basin upstream of the
overflow weir. Plant operating data are provided in the Table 14. From this
data it is seen that fluoride ion concentration is reduced from 1.13 mg/L
in the raw feed water to 0.504 mg/L in the clarifier overflow, and B
concentration is reduced from 0.741 to
TABLE 14
______________________________________
BARRICK GOLDSTRIKE MINES INC.
BOULDER VALLEY WATER TREATMENT PLANT
PLANT OPERATING DATA
Sample ID pH F B Mg Si Ca
______________________________________
Raw Feed Water
8.16 1.13 0.741
23.3 19.9 59.7
Raw Water with MgSO.sub.4
8.15 1.18 0.766
34.1 20.4
62.1
Outlet from Reactor
10.42
0.513 0.353
8.3 4.6
1.5
R-1 (after lime
addition)
Outlet from Reactors
10.5 0.486 0.376
6.4 3.0
1.6
R-2
Clarifier Overflow
10.73
0.504 0.506
4.5 3.5
3.6
Clarifier Underflow
10.56
0.477 0.465
156.2
0.6 8.7
After Neutralization
7.42 0.504 0.513
5.3 3.8
9.8
Plant Discharge
7.43
0.530 0.527
5.9 4.0
11.7
(after cooling
towers)
MgSO.sub.4 Stock Solution
8.66 0.465 0.684
5.4% 12.4
967.0
______________________________________
Note: 1) Units in mg/L except pH and as noted.
2) All data refers to solutions, no solids.
3) Plant flowrate 65,000 gpm
4) The pH and TDS of the clarifier overflow after neutralization were
within environmental guidelines.
EXAMPLE 8
A quantity of sludge generated in full scale operation of the water
treatment process of the invention was examined by scanning electron
microscope (SEM). From photomicrographs (FIGS. 13 to 16) it is concluded
that the precipitate formed by the process is in the nature of spherulitic
matter. On the basis of analyses including Time of Flight SIMS (Secondary
Ion Mass Spectrometry), Time of Flight LIMS (Laser Ionization Mass
Spectrometry), and Electron Probe Analysis, it is concluded that the
precipitated crystals have the structure presented schematically in FIG.
17.
As various changes could be made in the above embodiments without departing
from the scope of the invention, it is intended that all matter contained
in the above description shall be interpreted as illustrative and not in a
limiting sense.
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